Micro-optical elements from optical-quality ZIF-62 hybrid glasses by hot imprinting

Hybrid glasses derived from meltable metal-organic frameworks (MOFs) promise to combine the intriguing properties of MOFs with the universal processing ability of glasses. However, the shaping of hybrid glasses in their liquid state – in analogy to conventional glass processing – has been elusive thus far. Here, we present optical-quality glasses derived from the zeolitic imidazole framework ZIF-62 in the form of cm-scale objects. These allow for in-depth studies of optical transparency and refraction across the ultraviolet to near-infrared spectral range. Fundamental viscosity data are reported using a ball penetration technique, and subsequently employed to demonstrate the fabrication of micro-optical devices by thermal imprinting. Using 3D-printed fused silica templates, we show that concave as well as convex lens structures can be obtained at high precision by remelting the glass without trading-off on material quality. This enables multifunctional micro-optical devices combining the gas uptake and permeation ability of MOFs with the optical functionality of glass. As an example, we demonstrate the reversible change of optical refraction upon the incorporation of volatile guest molecules.

innovative.Nevertheless, it would be valuable to elucidate the practical applications of such devices.Can you provide an example to illustrate their potential utility? 2. In Fig. 3f, it is evident that the optical properties of agZIF-62, specifically the refractive index and Abbe value, are ordinary and comparable to those of polymers.Thus, it does not seem to be an ideal candidate for micro-optical elements.To provide clarity, please elaborate a bit more on the motivation behind this work.3.This work exclusively presents limited optical properties of the MOF glass microlenses.However, there is no information provided regarding their refractive and diffractive properties.Could you elaborate on these aspects to provide a more comprehensive understanding of the microlenses' optical characteristics? 4. In the abstract section, the assertion that the "shaping of hybrid glasses in their liquid state has been elusive thus far" seems inappropriate.Numerous prior studies have documented the successful preparation of bulk MOF glass.Could you revise this statement to align with the existing literature on the subject? 5. What causes the extensive generation of cracks around the concave or convex lenses, as observed in Fig. 1c, 1d, Fig. 5b, and 5c?Additionally, are there any proposed methods to mitigate the occurrence of these cracks?Addressing this issue is crucial as cracks in the lens can significantly degrade optical performance.6. Considering the high viscosity of the ZIF-62 melt, is the current melt-pressing approach capable of effectively removing bubbles from the melt?Please describe this aspect in the manuscript.7. On page 8, in the first paragraph, the authors attribute the shoulder peak at 290-300 nm to the unspecified energy levels of the agZIF-62 glass.Please give additional evidence or discussion to support this assignment.
Reviewer #4 (Remarks to the Author): This paper reports the fabrication of high-quality optical microlenses from a hybrid glass (ag-ZIF62) using classical glass forming techniques.This is made possible by initially synthesizing ag-ZIF62 a unprecedented material quality (homogeneity, optical transparency).While ag-ZIF62 has become something like the archetype hybrid glass, sample material with similar quality was previously not available.As a great step forward, such high-quality material now enables a great leap also in the quality of property data; in the context of optical devices, UV-NIR spectroscopy, optical refraction and optical dispersion are presented.More compellingly, direct viscosity data are also obtained for the first time by a ball penetration technique, demonstrating equilibrium as well as non-equilibrium viscosity.From such viscosity data, real-world processing windows can be defined within which hot glass forming is possible.Specifically, microlenses are generated by hot imprinting, and subsequently examined for their optical properties.Overall, the paper does not only demonstrate the use of a hybrid glass as a real-world optical material (for example, for use in responsive optical devices).More importantly, it demonstrates for the first time that the intriguing properties of a MOF can indeed be combined with the universal and facile hot glass forming ability; a promise which had previously stipulated intense research in the growing field of MOF glasses, but was never fulfilled until now.
(1) In Figure 3b, there is still some loss at near-infrared waveband region.What is the origin of these absorptions?
(2) The authors demonstrate the potential inactive optical applications.Can this new glass can host active dopants and has potentials for active photonics?For examples, as gain materials for optical amplifier and laser (Adv. Sci. 2023, 10, 2303421;Adv. Opt. Mater., 2021, 9, 2101394.)?
(3) How about the optical quality of the convex lenses in Figure 4?
We would like to thank the four expert reviewers for taking the time to consider our work in detail.The current revision was done in answer to the comments and suggestions provided by the reviewers as well as during the appeal process.Below, is a step-by-step response as to how the manuscript was amended in answer to each comment.

Reviewer #1 (Remarks to the Author):
In the present manuscript, the authors report a ZIF-62-Zn glass.The glass is formed by hot imprinting.We thank the Reviewer for their �me and considera�on, but must respec�ully disagree with the statements, emphasizing the lack of novelty in our work.For that, we refer to our arguments which have been already formulated during the appeal process:

The works lacks novelty for a broad community. The shaping, casting, forming and hot-pressing
From the appeal letter to the editor (09.02.24): "Carefully reading the reviewer comments, we acknowledge that the highly multidisciplinary nature of our manuscript -and the fact that hybrid glass research, on the contrary, is still mostly done in the field of chemistry -might have made the review process challenging.This might have led to misunderstandings, which we aim to resolve in the following.We do not think, however, that the multidisciplinary readership of NComms would face a similar issue.On the contrary, we are convinced that the current manuscript will make the field of hybrid glasses significantly more accessible for researchers, engineers and technologists outside of the immediate field of materials chemistry in that it showcases the real-world value of hybrid glasses.This could provide great opportunities; it would stipulate intense, application-oriented efforts beyond materials chemistry while, at the same time, being further motivation and justification for extended research across the material sciences.

[…]
MOF-glass objects intentionally shaped through hot glass forming and, in particular, micro-optical devices such as lenses or structured surfaces are not known to us and are not reported -to our best knowledge -in current literature.
Reviewer 1 -in their very short comment -argues that "shaping, casting, forming and hot-pressing" (of MOF-glasses) are state of the art.We respectfully disagree with this statement in that it does not refer to conventional glass processing (the great MOF promise).Of the three papers mentioned by this reviewer to substantiate their claim, we would limit our initial response to the review paper published in Nov. 2023, Adv. Funct. Mater., https://onlinelibrary.wiley.com/doi/full/10.1002/adfm.202307226 : Chapter 3.3 of this paper is devoted to "Morphological Control" of MOF/COF glasses.This chapter explicitly mentions that (conventional glass processing) "is hindered by the relatively narrow temperature window between Tm and Td" (the melting and the decomposition temperature of the MOF).Instead, the authors argue that alternative approaches are required, and mention vacuumassisted sintering or re-melting (but noting that the latter typically leads to poor sample quality).Figure 6 provides good examples of the kind of samples currently obtained in this way; we believe that the issues with sample quality are obvious in these photographs.In addition, the review mentions the ability to "mold cast" certain coordination polymers (ZnPlm); a photograph of such a material is provided in Fig. 3C: it is clearly opaque-white and certainly not a typical glass.We argue that not only do we prove that conventional glass processing is indeed possible, the sample quality obtained in our case is indeed a game-changer towards real-world application (a showcase example of this very glass is provided, on another note, through our recent contribution to the cover illustration of Nat.Mater.'s February issue1 ).Furthermore, we would highlight that any alternative processing technique involving pronounced pressure gradients (such as mentioned in the above review paper, but also used in the past by ourselves) is counter-productive in that it leads to a well-known loss of MOF porosity.
The second paper mentioned by this reviewer (Cell Reports Physical Sciences 2022, https://doi.org/10.1016/j.xcrp.2022.100932 ) discusses powder-sintering of MOF glasses, a subject we find unrelated to our work.The sample quality obtained in this latter study is shown in Fig. 1d of this paper; it appears out of question to us that this material does not establish novelty over our current demonstration, both in terms of sample quality and size.In the same paper, the authors also reproduce a photograph of one of our own samples from an earlier study, their Fig. 2c (our NComms 10.1038/s41467-021-25970-0 ).But this material, again, is not only deeply coloured as a result of being an IL/MOF composite containing decomposition products, it is also in no way a demonstration of any glass forming (shaping) technology.

[…]
Given the above, we assume that Reviewers 1 and 2 might have misunderstood our study in terms of its technological focus (but rather, they might have understood it as being focused on the physical aspects of glass formation and the making of a monolith).A demonstration of glass forming ability (in the technological sense) requires shaping in the hot state, without causing the object to degrade chemically, for example, by partial decomposition.We are convinced that we have made this demonstration in our manuscript, but agree over the potential dual meaning of the term "glass forming" (in the physical sense, leading to a glassy state, and in the technological sense, shaping the glass).

[…]
For one, repeating our arguments above, we provide evidence for conventional hot processing of MOFderived glasses.This confirms the material's promise, which has been around for several years by now, but remained largely unproven.Secondly, being prominently present in NComms, the subject field of hybrid glasses has been devoted almost exclusively to fundamental exploration of the glassy state and some of the achievable properties of monoliths or membranes.The demonstration of real-world processing "into something useful" will stipulate new interest across this community and beyond, hopefully at some point letting us harvest the fruits of all what has been achieved so far with this new class of glasses." We would also like to add that a�er the thorough revision process, our manuscript now offers another indisputable novelty in the field of hybrid MOF-glasses -the op�cal breathing effect, based on the influence of the guest molecules, adsorbed by the microporous matrix, on the op�cal proper�es -par�cularly the refrac�ve index.This process is reversible, causes no damage to the material, and is highly promising for real-world applica�ons, i.e. responsive op�cal elements for sensing devices, detec�ng contamina�ons in the ambient atmosphere.Moreover, we now demonstrate the ability of MOF-glass to be polished to achieve op�cal smoothness.Here we present the new chapter and corresponding addi�ons to Methods and Supplementary Informa�on: Manuscript:

Microporous agZIF-62 for responsive micro-optics
In real-world applications the surface of an optical element usually must be optically smooth, that is, its roughness should be considerably smaller than the wavelength of incident light.Then, surface scattering becomes neglectable, which is especially important in the case of micro-scale optical devices.When a device is manufactured by hot-imprinting or molding, its surface roughness will be strongly affected by the texture of the mold, and using smooth templates is beneficial. [48]Otherwise, the ability of a material to achieve a fine finish by polishing is required.Therefore, despite the well-known softness of MOF-glasses compared to oxide glasses [49] , we show that agZIF-62 still can be polished in a straightforward way.The surface of agZIF-62 pieces was polished with 1 µm diamond polishing spray on a rotating polishing felt, and the resulting surface roughness was determined by atomic force microscopy (AFM) (Figure 6a).With the maximal deviations staying within the nanometer-range, the average surface roughness Ra was determined to be less than 5 nm.Considering that mechanical polishing of soft materials is not a primary focus of the current report, achieving such low roughness values in such a facile approach is a strong indicator for the suitability of MOF-glasses for optical polishing.
In Figure 6b we demonstrate that the introduction of guests into the pores of MOF-glass can be utilized to significantly change the optical properties.The optical path length in agZIF-62 was measured with white light after being exposed to the different mediums, and the respective refractive indices were calculated (see Methods, Figures S4-S5).First, the optical path length was determined for pre-activated sample.Then, exposure to dichloromethane (DCM), and subsequently to methanol (MeOH) led to an increase in the refractive index by ~1.26% and ~1.33%, respectively.The initial refractive index (and optical path length) was fully recovered through re-activation of the sample by heating at low pressure.To this end, we demonstrate that adsorbed volatile molecules influence the refractive index of the system, and that this process is reversible.Therefore, the optical breathing effect [47] can be generated in agZIF-62 and similar MOF-derived glasses.Moreover, neither activation nor exposure to the selected solvents damaged the material on the macro-scale, as it remained visually identical between the experiments (Figure S6).This result paves a way for realizing responsive optical elements based on microporous MOF-derived glasses, e.g., stimulus-responsive micro-lenses becoming parts of optical sensing devices (Figure 6c).

Atomic force microscopy (AFM)
Prior to the measurement, the sample was glued to a glass slide and wet ground with water using 1000 grit paper and subsequently polished with 1 µm diamond polishing spray on a soft polishing felt.Surface topography of the polished glass was characterized using a commercial AFM system (Dimension Edge, Bruker).Measurement was carried out in Tapping Mode, using a silicon tip with a radius of 8 nm and a drive frequency of 209 kHz.Individual measurements were taken at multiple positions on the surface with different sizes ranging from 5 x 5 µm2 to 100 x 100 µm2.Data processing was done using the free Gwyddion software package, v2.65.Post-processing was limited to data leveling and subtraction of a polynomial background of 2nd degree to remove surface waviness, prior to statistical analysis within the software to derive the roughness characteristics.S4).First, the sample was pre-activated (150°C, 20 mbar, 24 hours) to get rid of the possible volatile non-air molecules, and the optical path length was evaluated.Then the microporous glass was left soaking in dichloromethane (DCM) for two hours, dried on the surface, and tested again.After that, the sample was exposed to methanol (MeOH) for 2 hours and re-activated, followed by the optical path length determination after each step.The images were collected in coaxial white light from the bottom of the sample (diffraction grating) to the very sharp image of the grating in the sample without reaching the surface to avoid artifacts.Five 2D profiles were collected for each experiment, processed in an identical way, and approximated by concatenate linear fit (Figure S5).The differences in depths were determined at the edge of the sample, and the value was subtracted from the geometrical sample thickness, resulting in the optical path lengths shown in Figure 6b of the main text.Refractive indices were calculated by dividing the sample thickness (i.e.geometrical optical path length) by the obtained optical path length.The errors were calculated based on the average absolute deviations of the profiles from their concatenate linear approximation.

Figure S5
2D profiles used for the optical path length determination in pre-activated, soaked in DCM and MeOH, and re-activated agZIF-62 of 147.4 µm thickness; their corresponding linear approximations and calculated depths.

Figure S6
Optical photographs of the same pre-activated, soaked in DCM and MeOH, and re-activated agZIF-62 piece.The piece remains unchanged (besides the glass dust on the surface of pre-activated sample, which was washed away in the solvent while soaking).

Oksana et al, prepared ZIF-62 glass in the shape of convex and concave structures by thermal imprinting using 3D fused silica template. However, the novelty and importance are poor. I don't think this draft is suitable for Nature Communications and I recommend the transfer of this manuscript to Scientific reports.
We thank the referee for the �me dedicated to collec�ng valuable comments, which are to be discussed below.However, as in the case of the Reviewer 1, we believe that the impression of the lack of novelty in our work might have been caused by the mul�disciplinary nature of the manuscript, leading to confusion in terms.To support our posi�on, we would like to refer to our granted appeal again: From the appeal letter to the editor (09.02.24): "Carefully reading the reviewer comments, we acknowledge that the highly multidisciplinary nature of our manuscript -and the fact that hybrid glass research, on the contrary, is still mostly done in the field of chemistry -might have made the review process challenging.This might have led to misunderstandings, which we aim to resolve in the following.We do not think, however, that the multidisciplinary readership of NComms would face a similar issue.On the contrary, we are convinced that the current manuscript will make the field of hybrid glasses significantly more accessible for researchers, engineers and technologists outside of the immediate field of materials chemistry in that it showcases the real-world value of hybrid glasses.This could provide great opportunities; it would stipulate intense, application-oriented efforts beyond materials chemistry while, at the same time, being further motivation and justification for extended research across the material sciences.

[…]
MOF-glass objects intentionally shaped through hot glass forming and, in particular, micro-optical devices such as lenses or structured surfaces are not known to us and are not reported -to our best knowledge -in current literature.

[…]
Given the above, we assume that Reviewers 1 and 2 might have misunderstood our study in terms of its technological focus (but rather, they might have understood it as being focused on the physical aspects of glass formation and the making of a monolith).A demonstration of glass forming ability (in the technological sense) requires shaping in the hot state, without causing the object to degrade chemically, for example, by partial decomposition.We are convinced that we have made this demonstration in our manuscript, but agree over the potential dual meaning of the term "glass forming" (in the physical sense, leading to a glassy state, and in the technological sense, shaping the glass).

[…]
For one, repeating our arguments above, we provide evidence for conventional hot processing of MOFderived glasses.This confirms the material's promise, which has been around for several years by now, but remained largely unproven.Secondly, being prominently present in NComms, the subject field of hybrid glasses has been devoted almost exclusively to fundamental exploration of the glassy state and some of the achievable properties of monoliths or membranes.The demonstration of real-world processing "into something useful" will stipulate new interest across this community and beyond, hopefully at some point letting us harvest the fruits of all what has been achieved so far with this new class of glasses." We would also like to add that a�er the thorough revision process, our manuscript now offers another indisputable novelty in the field of hybrid MOF-glasses -the op�cal breathing effect, based on the influence of the guest molecules, adsorbed by the microporous matrix, on the op�cal proper�es -par�cularly the refrac�ve index.This process is reversible, causes no damage to the material, and is highly promising for real-world applica�ons, i.e. responsive op�cal elements for sensing devices, detec�ng contamina�ons in the ambient atmosphere.

More specific comments are as follows:
-ZIF-62 glass is interesting material because of its porosity.However, ZIF-62 glass is not suitable for optical applications because of its poor mechanical properties.For example, the fracture toughness of ZIF-62 glass is 0.1 MPa, which is even lower than that of brittle oxide glasses due to the weak coordinative bonds (Zn-N) (Nat Commun 11, 2593, 2020).The low hardness of ZIF-62 glass prohibits this kind of materials for optical applications.
We thank the Reviewer for this comment, which, among others, has mo�vated us to demonstrate more convincing proofs of the applicability of ZIF-62 glass in the field of op�cs.But first, we would like to refer to the appeal: From the appeal letter to the editor (09.02.24): "We must respectfully but strongly disagree with this statement, as there is no direct connection between mechanical properties and the ability to be utilized for optical applications.Not only inorganic glasses, but also a wide range of polymers or even liquids are used as optical components.Fracture toughness or hardness, as questioned by the Reviewer, is often not a primary factor in materials selection; although it is worth noting, that for the ellipsometry measurements ZIF-glasses have been successfully mechanically polished without overall damage to the samples.Finally, the Reviewer refers to a value of fracture toughness reported in literature (they surely intend to use the unit "MPa.m-1/2").The reviewer is surely aware of the sample quality used in the particular study mentioned above, and of the fact that mechanical studies very strongly rely on sample quality." In addi�on, now our work contains the AFM data of agZIF-62 sample, which has been manually polished to op�cal smoothness with rela�ve ease.Once again, this proves that mechanical proper�es of the material -if properly produced and treated -are sufficient for many needs.Some complica�ons in handling of MOF-glass at its early-development stage, when compared to the conven�onal glasses, are compensated by what this material offers in contrast, par�cularly tunable, guest-accessible porosity.

-The high transparency of ZIF-62 glass is already achieved in other reports (Optics Letters 44, 1623-1625, 2019). Can you retain this high transparency for higher thickness samples (e.g., 3-5 mm thickness)? To really embed ZIF-62 glass in optical applications, you should fabricate highly transparent ZIF-62 glass in different shapes with different thickness.
The work men�oned by the Reviewer indeed presents the transparency data for ZIF-62-derived glass and is acknowledged in our manuscript.However, a hot-pressing technique, u�lized in the men�oned work, with the pressure of 50 MPa applied to the material inevitably leads to trading-off porosity -one of the main benefits of MOF-glass.We believe that our work, among others, lays the founda�on for manufacturing MOF-glass in different shapes, most importantly, without significant losses in quality or porosity.This work, however, is focused on the micro-scale shaping, which has not been demonstrated before.

-The authors claim that they prepared high quality printed ZIF-62 glass, however, in Figure 1c, the SEM image of the printed ZIF-62 glass shows several cracks, reducing the glass quality. Can you prepare ZIF-62 glass in the shape of micro-lens? If the authors really want to prepare ZIF-62 glass in different shapes, other technique should be used rather than the thermal printing. I suggest to use injection molding technology for shaping ZIF-62 glass. The thermal printing technique results in the formation of ZIF-62 glass slide contains some curved structures, while, it is difficult to isolate these structures because of the low hardness of ZIF-62 glass.
We no�ce that the Reviewer uninten�onally mixed up the figures and their cap�ons, which has led to confusion.This issue has already been discussed in detail: From the appeal letter to the editor (09.02.24): "Figure 1c is a SEM image of the template (not the glass!), which was used for thermal imprinting; a part of the same template is also shown in Figure 4e.The actual glass (with the imprints) is in Figure 1d, and it is crack-free.Although the cracking problem is obvious for the templates (produced by 2-Photon-Lithography using a silica precursor), this is not in the scope of the current study.Instead, we demonstrate that even small defects (on the template) can be transferred to the glass substrate by thermal imprinting, indicating the very high possible spatial resolution of this technique.
Furthermore, we find that this Reviewer is in error suggesting the use of injection molding as an alternative hot glass forming technique.Any technical issues notwithstanding, we argue in the manuscript that processing involving high pressures would trade-off MOF porosity -the very reason why using MOF-glasses in the first place."

-What is the practical application of the printed ZIF-62 glass? Can you control the refractive index of the glass?
We would like to thank the Reviewer for this comment, as it inspired us to test the op�cal breathing effect in our material -an idea that we introduced as a perspec�ve in our original manuscript but did not risk approaching beforehand due to the presump�ve complexity.We realize that addi�onal data would help the readers of our paper to understand this unique func�onality of hybrid, MOF-derived glasses in a more instruc�ve way.Therefore, Figure 6b and 6c demonstrate such new data, along with the following new paragraphs, addi�ons to the Methods sec�on, and Supplementary Informa�on: Manuscript:

Microporous agZIF-62 for responsive micro-optics […]
In Figure 6b we demonstrate that the introduction of guests into the pores of MOF-glass can be utilized to significantly change the optical properties.The optical path length in agZIF-62 was measured with white light after being exposed to the different mediums, and the respective refractive indices were calculated (see Methods, Figures S4-S5).First, the optical path length was determined for pre-activated sample.Then, exposure to dichloromethane (DCM), and subsequently to methanol (MeOH) led to an increase in the refractive index by ~1.26% and ~1.33%, respectively.The initial refractive index (and optical path length) was fully recovered through re-activation of the sample by heating at low pressure.To this end, we demonstrate that adsorbed volatile molecules influence the refractive index of the system, and that this process is reversible.Therefore, the optical breathing effect [47] can be generated in agZIF-62 and similar MOF-derived glasses.Moreover, neither activation nor exposure to the selected solvents damaged the material on the macro-scale, as it remained visually identical between the experiments (Figure S6).This result paves a way for realizing responsive optical elements based on microporous MOF-derived glasses, e.g., stimulus-responsive micro-lenses becoming parts of optical sensing devices (Figure 6c).

Transparent piece of agZIF-62 was placed on the diffraction grating (Carl Zeiss Jena, 20 lines per mm).
A digital microscope (VHX-6000, Keyence) with a universal zoom lens (VH-Z100UR and VHX-S650) was utilized to obtain 3D images of the edge of agZIF-62 piece with different incorporated guest molecules (Figure S4).First, the sample was pre-activated (150°C, 20 mbar, 24 hours) to get rid of the possible volatile non-air molecules, and the optical path length was evaluated.Then the microporous glass was left soaking in dichloromethane (DCM) for two hours, dried on the surface, and tested again.After that, the sample was exposed to methanol (MeOH) for 2 hours and re-activated, followed by the optical path length determination after each step.The images were collected in coaxial white light from the bottom of the sample (diffraction grating) to the very sharp image of the grating in the sample without reaching the surface to avoid artifacts.Five 2D profiles were collected for each experiment, processed in an identical way, and approximated by concatenate linear fit (Figure S5).The differences in depths were determined at the edge of the sample, and the value was subtracted from the geometrical sample thickness, resulting in the optical path lengths shown in Figure 6b of the main text.Refractive indices were calculated by dividing the sample thickness (i.e.geometrical optical path length) by the obtained optical path length.The errors were calculated based on the average absolute deviations of the profiles from their concatenate linear approximation.

Figure S5
2D profiles used for the optical path length determination in pre-activated, soaked in DCM and MeOH, and re-activated agZIF-62 of 147.4 µm thickness; their corresponding linear approximations and calculated depths.

Figure S6
Optical photographs of the same pre-activated, soaked in DCM and MeOH, and re-activated agZIF-62 piece.The piece remains unchanged (besides the glass dust on the surface of pre-activated sample, which was washed away in the solvent while soaking).

-The authors should show whether the printed structures contain bubbles. For optical applications, the surface roughness of optical lens should be less than Ra<0.05 μm. The authors should also measure surface roughness (Ra) for the printed glasses to determine whether it has high quality.
Glasses, presented in this work, have been prepared based on our earlier developed procedure (Nature Materials, 23, 262-270 (2024)), which leads to the material being defect-and bubble-free.The absence of bubbles can be observed within this work in Figures 4c,g, Figure 5a, Figure S6.Then, we would like to acknowledge the sugges�on of the Reviewer and present our valuable addi�on to the manuscriptsurface roughness of as-polished MOF-glass evaluated through AFM.Thus, Figure 6a and new paragraph, as well as some addi�ons to Methods: Manuscript:

Microporous agZIF-62 for responsive micro-optics
In real-world applications the surface of an optical element usually must be optically smooth, that is, its roughness should be considerably smaller than the wavelength of incident light.Then, surface scattering becomes neglectable, which is especially important in the case of micro-scale optical devices.When a device is manufactured by hot-imprinting or molding, its surface roughness will be strongly affected by the texture of the mold, and using smooth templates is beneficial. [48]Otherwise, the ability of a material to achieve a fine finish by polishing is required.Therefore, despite the well-known softness of MOF-glasses compared to oxide glasses [49] , we show that agZIF-62 still can be polished in a straightforward way.The surface of agZIF-62 pieces was polished with 1 µm diamond polishing spray on a rotating polishing felt, and the resulting surface roughness was determined by atomic force microscopy (AFM) (Figure 6a).With the maximal deviations staying within the nanometer-range, the average surface roughness Ra was determined to be less than 5 nm.Considering that mechanical polishing of soft materials is not a primary focus of the current report, achieving such low roughness values in such a facile approach is a strong indicator for the suitability of MOF-glasses for optical polishing. […]

Atomic force microscopy (AFM)
Prior to the measurement, the sample was glued to a glass slide and wet ground with water using 1000 grit paper and subsequently polished with 1 µm diamond polishing spray on a soft polishing felt.Surface topography of the polished glass was characterized using a commercial AFM system (Dimension Edge, Bruker).Measurement was carried out in Tapping Mode, using a silicon tip with a radius of 8 nm and a drive frequency of 209 kHz.Individual measurements were taken at multiple positions on the surface with different sizes ranging from 5 x 5 µm2 to 100 x 100 µm2.Data processing was done using the free Gwyddion software package, v2.65.Post-processing was limited to data leveling and subtraction of a polynomial background of 2nd degree to remove surface waviness, prior to statistical analysis within the software to derive the roughness characteristics.

This is an interesting work. It reports the fabrication of micro-optical elements using MOF glass. Both concave and convex lens structures were successfully produced on the surface of bulk MOF
glass through the thermal imprinting method.Subsequently, various optical properties of these micro-optical elements were thoroughly examined.Despite being the first report on micro-optical elements utilizing MOF glass, the authors did not sufficiently describe the significance and novelty of their study.This is important for meeting the standards of Nature Communications.
We thank the Reviewer for a posi�ve evalua�on of our work.Now, a�er thorough revision, this manuscript not only emphasizes the novelty and importance in a clearer way, but also contains a new proof of applicability in op�cs -we demonstrate the op�cal breathing effect, which will be further addressed below the comment 1. Par�cularly, aside from discussion sec�on, exci�ng perspec�ves of the material in the field of op�cs have been underlined in abstract and conclusions through following sentences: Abstract: Unique to hybrid glasses, this enables tailorable and stimulus-responsive optical performance, demonstrated by way of example through the reversible change of optical refraction upon the incorporation of volatile guest molecules.

Conclusion:
We demonstrated that utilizing the porous system remaining in the MOF-glasses, as their unique feature, enables responsive micro-optical devices that shift their optical properties in response to the adsorption and desorption of guest molecules; a new route towards stimulus-responsive optical sensing materials.

The integration of optical functionality and gas permeation in a glass device appears to be innovative. Nevertheless, it would be valuable to elucidate the practical applications of such devices. Can you provide an example to illustrate their potential utility?
We are grateful to the Reviewer for this comment, and hereby present an applicable dependence of the refrac�ve index on the guest molecules, incorporated into the nanoporous matrix of ZIF-62 glass.Figures 6b, new paragraphs in the main text and Methods, and Supplementary Informa�on Figures have been added (the ability of MOF-glass to be polished to achieve op�cal smoothness is also now demonstrated on the same Figure ):

Microporous agZIF-62 for responsive micro-optics
In real-world applications the surface of an optical element usually must be optically smooth, that is, its roughness should be considerably smaller than the wavelength of incident light.Then, surface scattering becomes neglectable, which is especially important in the case of micro-scale optical devices.When a device is manufactured by hot-imprinting or molding, its surface roughness will be strongly affected by the texture of the mold, and using smooth templates is beneficial. [48]Otherwise, the ability of a material to achieve a fine finish by polishing is required.Therefore, despite the well-known softness of MOF-glasses compared to oxide glasses [49] , we show that agZIF-62 still can be polished in a straightforward way.The surface of agZIF-62 pieces was polished with 1 µm diamond polishing spray on a rotating polishing felt, and the resulting surface roughness was determined by atomic force microscopy (AFM) (Figure 6a).With the maximal deviations staying within the nanometer-range, the average surface roughness Ra was determined to be less than 5 nm.Considering that mechanical polishing of soft materials is not a primary focus of the current report, achieving such low roughness values in such a facile approach is a strong indicator for the suitability of MOF-glasses for optical polishing.
In Figure 6b we demonstrate that the introduction of guests into the pores of MOF-glass can be utilized to significantly change the optical properties.The optical path length in agZIF-62 was measured with white light after being exposed to the different mediums, and the respective refractive indices were calculated (see Methods, Figures S4-S5).First, the optical path length was determined for pre-activated sample.Then, exposure to dichloromethane (DCM), and subsequently to methanol (MeOH) led to an increase in the refractive index by ~1.26% and ~1.33%, respectively.The initial refractive index (and optical path length) was fully recovered through re-activation of the sample by heating at low pressure.To this end, we demonstrate that adsorbed volatile molecules influence the refractive index of the system, and that this process is reversible.Therefore, the optical breathing effect [47] can be generated in agZIF-62 and similar MOF-derived glasses.Moreover, neither activation nor exposure to the selected solvents damaged the material on the macro-scale, as it remained visually identical between the experiments (Figure S6).This result paves a way for realizing responsive optical elements based on microporous MOF-derived glasses, e.g., stimulus-responsive micro-lenses becoming parts of optical sensing devices (Figure 6c).

Atomic force microscopy (AFM)
Prior to the measurement, the sample was glued to a glass slide and wet ground with water using 1000 grit paper and subsequently polished with 1 µm diamond polishing spray on a soft polishing felt.Surface topography of the polished glass was characterized using a commercial AFM system (Dimension Edge, Bruker).Measurement was carried out in Tapping Mode, using a silicon tip with a radius of 8 nm and a drive frequency of 209 kHz.Individual measurements were taken at multiple positions on the surface with different sizes ranging from 5 x 5 µm2 to 100 x 100 µm2.Data processing was done using the free Gwyddion software package, v2.65.Post-processing was limited to data leveling and subtraction of a polynomial background of 2nd degree to remove surface waviness, prior to statistical analysis within the software to derive the roughness characteristics.

Optical path length determination
Transparent piece of agZIF-62 was placed on the diffraction grating (Carl Zeiss Jena, 20 lines per mm).A digital microscope (VHX-6000, Keyence) with a universal zoom lens (VH-Z100UR and VHX-S650) was utilized to obtain 3D images of the edge of agZIF-62 piece with different incorporated guest molecules (Figure S4).First, the sample was pre-activated (150°C, 20 mbar, 24 hours) to get rid of the possible volatile non-air molecules, and the optical path length was evaluated.Then the microporous glass was left soaking in dichloromethane (DCM) for two hours, dried on the surface, and tested again.After that, the sample was exposed to methanol (MeOH) for 2 hours and re-activated, followed by the optical path length determination after each step.The images were collected in coaxial white light from the bottom of the sample (diffraction grating) to the very sharp image of the grating in the sample without reaching the surface to avoid artifacts.Five 2D profiles were collected for each experiment, processed in an identical way, and approximated by concatenate linear fit (Figure S5).The differences in depths were determined at the edge of the sample, and the value was subtracted from the geometrical sample thickness, resulting in the optical path lengths shown in Figure 6b of the main text.Refractive indices were calculated by dividing the sample thickness (i.e.geometrical optical path length) by the obtained optical path length.The errors were calculated based on the average absolute deviations of the profiles from their concatenate linear approximation.

Figure S4
An example of dataset for optical path lenght determination collected by z-axis scanning using digital microscope: 3D image of the edge of agZIF-62 piece formed by z-axis scans stacking (top left), 2D profiles collected to determine the difference in depth (top right), an example of a profile (bottom).

Figure S5
2D profiles used for the optical path length determination in pre-activated, soaked in DCM and MeOH, and re-activated agZIF-62 of 147.4 µm thickness; their corresponding linear approximations and calculated depths.

Figure S6
Optical photographs of the same pre-activated, soaked in DCM and MeOH, and re-activated agZIF-62 piece.The piece remains unchanged (besides the glass dust on the surface of pre-activated sample, which was washed away in the solvent while soaking).

In Fig. 3f, it is evident that the optical properties of agZIF-62, specifically the refractive index and Abbe value, are ordinary and comparable to those of polymers. Thus, it does not seem to be an ideal candidate for micro-optical elements. To provide clarity, please elaborate a bit more on the motivation behind this work.
Referring to our response to comment 1 and the new data, we believe that now we have proved that tunable and accessible porosity offers excep�onal benefits, unique to this new class of glasses.

This work exclusively presents limited optical properties of the MOF glass microlenses. However, there is no information provided regarding their refractive and diffractive properties. Could you elaborate on these aspects to provide a more comprehensive understanding of the microlenses' optical characteristics?
In the manuscript, we indeed provide refrac�ve index and op�cal dispersion data, now complemented with further data demonstra�ng the variability of refrac�ve index upon the reversible introduc�on of guest molecules into the glass' pores.In this manuscript, however, we did not aim for diffrac�ve op�cs; we believe that this goes beyond the scope of a single paper.The imprinted lenses are intended "only" a demonstra�on of the general ability to conven�onally shape the material through the hot-imprin�ng technique.We found it important to show that this process preserves the high op�cal quality of the original glass, and that the obtained object can indeed be used as lenses, as proven by demonstra�ng the magnifying ability.However, a more specialized inves�ga�on of the lenses' proper�es was not in the scope of the current manuscript (being intended for a more general audience) and would distract readers from its central objec�ve.

In the abstract section, the assertion that the "shaping of hybrid glasses in their liquid state has been elusive thus far" seems inappropriate. Numerous prior studies have documented the successful preparation of bulk MOF glass. Could you revise this statement to align with the existing literature on the subject?
Here we must emphasize that the mul�disciplinary nature of the work might have led to the confusion in terms.Within this manuscript, we refer to "glass shaping" as a technological process, which includes the treatment of a previously manufactured glass in a hot state, and not as a process of bulk glass fabrica�on through mel�ng.As the dual meaning of "glass shaping/forming" -technological and physical -might lead to further confusion, we now refer to it as "conven�onal glass shaping", which is only atributed to a technological process.The following sentence was changed from: However, the shaping of hybrid glasses in their liquid state has been elusive thus far.

To:
However, the shaping of hybrid glasses in their liquid state -in analogy to conventional glass processing -has been elusive thus far.

What causes the extensive generation of cracks around the concave or convex lenses, as
observed in Fig. 1c, 1d, Fig. 5b, and 5c?Additionally, are there any proposed methods to mitigate the occurrence of these cracks?Addressing this issue is crucial as cracks in the lens can significantly degrade optical performance.
Going one by one, Figure 1c only shows a 3D-printed template used for hot-imprin�ng, i.e. a nega�ve of the lens.Although the cracking, caused by the not-yet-op�mized fabrica�on process, is obvious, it neither affects the lenses of the template nor the imprinted structures in MOF-glass.Both template's and glass's lenses are finely formed, smooth, and symmetrical, as shown by LSM in Figures 5 c,d,e.
Figure 1d does show ZIF-62 glass with the imprinted structures, but there are no cracks caused by the surface micro-structuring.Aside from the edge of the sample (which was selected in a small size to fit into the measurement cell), all space between the lenses is smooth, crack-and defect-free, just like the lenses themselves (which is clearly visible in Figures 5 a,b).

Considering the high viscosity of the ZIF-62 melt, is the current melt-pressing approach capable of effectively removing bubbles from the melt? Please describe this aspect in the manuscript.
Glasses, presented in this work, have been prepared based on our earlier developed procedure (Nature Materials, 23, 262-270 (2024)), which leads to the material being defect-and bubble-free.Op�cal microphotographs of agZIF-62 pieces in a high resolu�on within this manuscript (par�cularly Figures 4 c,g, Figure 5a, Figure S6) do not show any evidence of the bubbles throughout the glass volume.
7. On page 8, in the first paragraph, the authors attribute the shoulder peak at 290-300 nm to the unspecified energy levels of the agZIF-62 glass.Please give additional evidence or discussion to support this assignment.
Previously unspecified men�oned energy levels are now atributed to the ligand-to-metal charge transfer (LMCT), as similar paterns have been observed for MOFs.The corresponding part is now corrected from: Other than previously reported [10], we find a few distinct shoulders in agZIF-62 slightly above the UV absorption edge (Figure 3a), which we attribute to discrete, as-of-yet unspecified energy levels of the hybrid glass.We assume that this finding is related to material purity and processing parameters; the present agZIF-62 did not undergo high-temperature/high-pressure densification upon melting.From the peak position and the sharp band at wavelength 290-300 nm, we can exclude that this band comes from a ZnO cluster, as these normally absorb in the regions above 350 nm [35,36].

To:
Other than previously reported [10] , we find a few distinct shoulders in agZIF-62 slightly above the UV absorption edge (Figure 3a), which we attribute to ligand-to-metal charge transfer [35,36] in accordance with the coordinating nature of bonds within the material, preserved after melting.We assume that this finding is related to material purity and processing parameters; the present agZIF-62 did not undergo high-temperature/high-pressure densification upon melting.

This paper reports the fabrication of high-quality optical microlenses from a hybrid glass (ag-ZIF62) using classical glass forming techniques. This is made possible by initially synthesizing ag-ZIF62 a unprecedented material quality (homogeneity, optical transparency). While ag-ZIF62 has become something like the archetype hybrid glass, sample material with similar quality was previously not available. As a great step forward, such high-quality material now enables a great leap also in the quality of property data; in the context of optical devices, UV-NIR spectroscopy, optical refraction and optical dispersion are presented. More compellingly, direct viscosity data are also obtained for the first time by a ball penetration technique, demonstrating equilibrium as well as non-equilibrium viscosity. From such viscosity data, real-world processing windows can be defined within which hot glass forming is possible. Specifically, microlenses are generated by hot imprinting, and subsequently examined for their optical properties. Overall, the paper does not only demonstrate the use of a hybrid glass as a real-world optical material (for example, for use in responsive optical devices). More importantly, it demonstrates for the first time that the intriguing properties of a MOF can indeed be combined with the universal and facile hot glass forming ability; a promise which had previously stipulated intense research in the growing field of MOF glasses, but was never fulfilled until now.
We are very grateful to the Reviewer for a posi�ve evalua�on of the work and for sharing our excitement on the topic.Below we will address all their comments individually.
(1) In Figure 3b, there is still some loss at near-infrared waveband region.What is the origin of these absorptions?
Losses at near-infrared region in Figure 3b (as well as reproduced transmitance patern in Figure 3d) are atributed to the fingerprint region of the material in the manuscript.We agree with the Reviewer that the demonstra�on of a use-case for our material is indeed important, and their sugges�ons seem promising and are worth inves�ga�ng.However, within this work we chose to focus on exploi�ng the materials' porosity as its main feature, as conven�onal doping can be performed with other glasses as well (which does not decrease a poten�al interest in a doped MOFglass).Now we demonstrate that nanoporous agZIF-62 can be u�lized in responsive micro-op�cs.Figures 6b,c, new paragraphs in the main text and Methods, and Supplementary Informa�on Figures have been added (the ability of MOF-glass to be polished to achieve op�cal smoothness is also now demonstrated on the same Figure ): Manuscript:

Microporous agZIF-62 for responsive micro-optics
In real-world applications the surface of an optical element usually must be optically smooth, that is, its roughness should be considerably smaller than the wavelength of incident light.Then, surface scattering becomes neglectable, which is especially important in the case of micro-scale optical devices.When a device is manufactured by hot-imprinting or molding, its surface roughness will be strongly affected by the texture of the mold, and using smooth templates is beneficial. [48]Otherwise, the ability of a material to achieve a fine finish by polishing is required.Therefore, despite the well-known softness of MOF-glasses compared to oxide glasses [49] , we show that agZIF-62 still can be polished in a straightforward way.The surface of agZIF-62 pieces was polished with 1 µm diamond polishing spray on a rotating polishing felt, and the resulting surface roughness was determined by atomic force microscopy (AFM) (Figure 6a).With the maximal deviations staying within the nanometer-range, the average surface roughness Ra was determined to be less than 5 nm.Considering that mechanical polishing of soft materials is not a primary focus of the current report, achieving such low roughness values in such a facile approach is a strong indicator for the suitability of MOF-glasses for optical polishing.
In Figure 6b we demonstrate that the introduction of guests into the pores of MOF-glass can be utilized to significantly change the optical properties.The optical path length in agZIF-62 was measured with white light after being exposed to the different mediums, and the respective refractive indices were calculated (see Methods, Figures S4-S5).First, the optical path length was determined for pre-activated sample.Then, exposure to dichloromethane (DCM), and subsequently to methanol (MeOH) led to an increase in the refractive index by ~1.26% and ~1.33%, respectively.The initial refractive index (and optical path length) was fully recovered through re-activation of the sample by heating at low pressure.To this end, we demonstrate that adsorbed volatile molecules influence the refractive index of the system, and that this process is reversible.Therefore, the optical breathing effect [47] can be generated in agZIF-62 and similar MOF-derived glasses.Moreover, neither activation nor exposure to the selected solvents damaged the material on the macro-scale, as it remained visually identical between the experiments (Figure S6).This result paves a way for realizing responsive optical elements based on microporous MOF-derived glasses, e.g., stimulus-responsive micro-lenses becoming parts of optical sensing devices (Figure 6c).

Atomic force microscopy (AFM)
Prior to the measurement, the sample was glued to a glass slide and wet ground with water using 1000 grit paper and subsequently polished with 1 µm diamond polishing spray on a soft polishing felt.Surface topography of the polished glass was characterized using a commercial AFM system (Dimension Edge, Bruker).Measurement was carried out in Tapping Mode, using a silicon tip with a radius of 8 nm and a drive frequency of 209 kHz.Individual measurements were taken at multiple positions on the surface with different sizes ranging from 5 x 5 µm2 to 100 x 100 µm2.Data processing was done using the free Gwyddion software package, v2.65.Post-processing was limited to data leveling and subtraction of a polynomial background of 2nd degree to remove surface waviness, prior to statistical analysis within the software to derive the roughness characteristics.

Optical path length determination
Transparent piece of agZIF-62 was placed on the diffraction grating (Carl Zeiss Jena, 20 lines per mm).A digital microscope (VHX-6000, Keyence) with a universal zoom lens (VH-Z100UR and VHX-S650) was utilized to obtain 3D images of the edge of agZIF-62 piece with different incorporated guest molecules (Figure S4).First, the sample was pre-activated (150°C, 20 mbar, 24 hours) to get rid of the possible volatile non-air molecules, and the optical path length was evaluated.Then the microporous glass was left soaking in dichloromethane (DCM) for two hours, dried on the surface, and tested again.After that, the sample was exposed to methanol (MeOH) for 2 hours and re-activated, followed by the optical path length determination after each step.The images were collected in coaxial white light from the bottom of the sample (diffraction grating) to the very sharp image of the grating in the sample without reaching the surface to avoid artifacts.Five 2D profiles were collected for each experiment, processed in an identical way, and approximated by concatenate linear fit (Figure S5).The differences in depths were determined at the edge of the sample, and the value was subtracted from the geometrical sample thickness, resulting in the optical path lengths shown in Figure 6b of the main text.Refractive indices were calculated by dividing the sample thickness (i.e.geometrical optical path length) by the obtained optical path length.The errors were calculated based on the average absolute deviations of the profiles from their concatenate linear approximation.

Figure S4
An example of dataset for optical path lenght determination collected by z-axis scanning using digital microscope: 3D image of the edge of agZIF-62 piece formed by z-axis scans stacking (top left), 2D profiles collected to determine the difference in depth (top right), an example of a profile (bottom).

Figure S5
2D profiles used for the optical path length determination in pre-activated, soaked in DCM and MeOH, and re-activated agZIF-62 of 147.4 µm thickness; their corresponding linear approximations and calculated depths.

Figure S6
Optical photographs of the same pre-activated, soaked in DCM and MeOH, and re-activated agZIF-62 piece.The piece remains unchanged (besides the glass dust on the surface of pre-activated sample, which was washed away in the solvent while soaking).
(3) How about the optical quality of the convex lenses in Figure 4? Unfortunately, we could not detect the magnifying ability of the convex lenses.This can be atributed to the asymmetry in the original templates' lenses (Figure 4 a,b), or to the resul�ng imprints being not deep enough (Figure 4 c,d, Figure S3).Both factors could make the determina�on of a focal point/image plane more complicated, and both factors could be in perspec�ve overcome by fabrica�ng alterna�ve templates.Using variable templates adapted to more specific applica�ons and use-cases is fundamentally enabled by the current demonstra�on.

Figure 1
Figure 1 a) 2D AFM image of the polished surface of agZIF-62 for surface roughness determination.b) Changes in optical path length and refractive index of microporous agZIF-62 influenced by different guest molecules.The error bars represent the average absolute deviations of the measured profiles from their concatenate linear approximation.c) Schematic representation of a perspective responsive micro-optical element based on the microporous agZIF-62.

Figure 2
Figure 2 a) 2D AFM image of the polished surface of agZIF-62 for surface roughness determination.b) Changes in optical path length and refractive index of microporous agZIF-62 influenced by different guest molecules.The error bars represent the average absolute deviations of the measured profiles from their concatenate linear approximation.c) Schematic representation of a perspective responsive micro-optical element based on the microporous agZIF-62.

Figure 3
Figure 3 a) 2D AFM image of the polished surface of agZIF-62 for surface roughness determination.b) Changes in optical path length and refractive index of microporous agZIF-62 influenced by different guest molecules.The error bars represent the average absolute deviations of the measured profiles from their concatenate linear approximation.c) Schematic representation of a perspective responsive micro-optical element based on the microporous agZIF-62.

Figure 4
Figure 4 a) 2D AFM image of the polished surface of agZIF-62 for surface roughness determination.b) Changes in optical path length and refractive index of microporous agZIF-62 influenced by different guest molecules.The error bars represent the average absolute deviations of the measured profiles from their concatenate linear approximation.c) Schematic representation of a perspective responsive micro-optical element based on the microporous agZIF-62.

( 2 )
The authors demonstrate the potential inactive optical applications.Can this new glass can host active dopants and has potentials for active photonics?For examples, as gain materials for optical amplifier and laser(Adv.Sci.2023, 10, 2303421; Adv.Opt.Mater., 2021, 9, 2101394.)?

Figure 5
Figure 5 a) 2D AFM image of the polished surface of agZIF-62 for surface roughness determination.b) Changes in optical path length and refractive index of microporous agZIF-62 influenced by different guest molecules.The error bars represent the average absolute deviations of the measured profiles from their concatenate linear approximation.c) Schematic representation of a perspective responsive micro-optical element based on the microporous agZIF-62.